Bayer method is generally used in factories for the production of aluminium-oxide. The produced hydroxide is a material in the form of fine mesh-grade ranges of aluminium hydroxide produced from the decomposer are rather wide, so that grading is needed to obtain sand-like aluminium hydroxide, coarse inoculating crystal and fine inoculating crystal. The sand-like aluminium hydroxide of coarse grades (more than 90% of +44 .mu.m particeles is fed to the next sintering station to form aluminium oxide, while the coarse crystals and fine crystals in the medium and fine grade are respectively fed back to the three-stage or one-stage decomposers for regrowing. At present, in order to perform grading of three products as hove mentioned, two-stage gravity subsiders or two-stage hydraulic cyclones are frequently used for the grading in many countries in the world. Such methods have the following deficiencies, i.e. low grading efficiency, bulky structure, complicated technological processes, high investment, high power comsumption, etc. In light of the above, in 1987 the same inventor developed a eyelone fine screen grading device, wherein a plastic cylindrical screen is installed into a hydraulic cyclone housing so as to integrate the hydraulic cyclone with the arc-shaped screen into one assembly. At the meantime, two grading principles, i.e. gravity centrifugation and fine-particle screening, are adopted to achieve the aim of increasing the efficiencies. The device consists of an overflow cap, an overflow pipe, a feeding section, a plurality of cylindrical sections, a plurality of cylindrical screens, a plurality of flanges, an undersized, section, a conical section, a lower outlet, and a pressure gauge, etc. The upper part is a small cylinder, consisting of the feeding section and the overflow cap, and being the input of material syrup and the overflow exhaust. The external portion of middle part is the plurality of cylindrical sections; the inner portion is the cylindrical screens, having the diameter corresponding to that of the upper small cylinder. The diameter of the plurality of cylindrical sections is 1.3 times of that of the upper small cylinder. The lower part is the undersized section, which is fixed with an exhaust for medium-sized particle product, the exhaust being perpendicular to the plurality of cylindrical sections. Passing through the undersized section, there is fixed a partially exposed, hollow inverted conical section. The cylindrical screen is made of nylon 1010, formed by injection moulding and pressing into a rectangular screen (475.65.times.150 mm). The meshes of the screen are in stripes, with three specifications, i.e. 0.15, 0.3 and 0.45 mm. The cross section of the screen stipes is in the form of wedges. The rectangular fine screen is further processed to form the cylindrical screen. Such cyclone screen device has been adopted in production and has been regareded to have a lot of advantages. However, it still has some deficiencies, i.e. the structure of fixing the cylindrical screens is unreasonable so that it is easy to be deformed or even be dropped-off under the working condition of high-speed cyclone flow and pressure inpact as well as a working temperature of nearly 60.degree. C. Thus, it has to be replaced frequently. Because of only one single specification of the rectangular screen as to length and width, different specifications (different diameters and different heights) of cyclone fine screen are supplied by butt jointing the rectangular screen in longitudinal and radial directions. For the butt joint in the height, same number of cylindrical sections are needed for the same number of screens. Moreover, the plurality of cylindrical sections are connected by flanges. Under such construction, during the replacement of the screen, a plurality cylindrical sections and flanges should be dismounted one by one, which brings difficulties to installation as well as maintenance and will influence the normal production.